Handle process.argv

[phated]

Liftoff should handle process.argv. Currently, the dance between yargs and Liftoff in gulp-cli is annoying and a little problematic. If Liftoff just managed the args, it would probably make for a nicer integration.

We could keep using yargs, but should explore https://nodejs.org/docs/latest/api/util.html#utilparseargsconfig since it would remove a dependency.